Recently, the feasibility of obtaining a very hot plasma from the unconfined D(T,n).alpha. reaction has been explored. According to the concept, a reaction is expected to take place by directing a laser beam onto the surface of a frozen pellet of deuterium and tritium (D-T). In this approach, laser light is to be absorbed by the surface of the frozen pellet in order to create a spherically converging shock wave in the pellet interior. The pressure of the imploding shock wave causes the density and temperature in the center of the pellet to rise and a reaction takes place thereby, resulting in the production of a copious quantity of neutrons and x-rays.
A reaction based upon the above concept is described in U.S. Pat. No. 3,624,239, to Arthur P. Fraas, issued Nov. 30, 1971, wherein a bath of molten lithium is swirled relative to a fixed container. The rotation of this liquid produces a deep vortex into which the fuel pellet is dropped from above. When it reaches the center of the vortex, the pellet is illuminated by a high-power glass laser fired from above. Because the vortex is narrow, most of the neutrons, charged particles and x-rays produced in the hot expanding D-T plasma are absorbed by the lithium. Only a slight amount of the radiation is expected to escape out of the top of the vortex. The reaction produces a shock wave in the lithium that is absorbed with the aid of gas bubbles dispersed in the lithium. The molten lithium breeds tritium through reactions such as Li.sup.6 (n,.alpha.)T and Li.sup.7 (n,.alpha.n')T, and is also pumped through a recirculating system where it serves as a heat transfer fluid, etc., and to a tritium recovery system.
Recently, one investigator examined the laser-pellet process in greater detail. In an article by Nuckolls et al. published in Nature 239 (1972) 139, it was reported that the laser pulse must be focused with a spatial uniformity in intensity of approximately 96%. This is for a pulse containing approximately 10.sup.5 J of about 10.sup.-10 sec. duration striking the surface of a typical pellet of 0.4 mm radius. If it is assumed that the foregoing is a correct requirement, unsymmetrical illumination of the pellet--as in Fraas' above system--will not be sufficient for achieving the desired reaction. The upper half of the pellet in Fraas' case would receive the full laser energy while the lower half would not be illuminated.
If the pellet were to be illuminated by six beams oriented along the six Cartesian directions as suggested by Nuckolls et al. in the above publication, 96% illumination still would not be achieved but this would be a close approximation. Nuckolls' design, however, does not provide liquid lithium reactor walls and it would be difficult to visualize how these six beams could be utilized in a container of liquid lithium.
In another approach to uniform illumination of the pellet, it might be proposed to employ some kind of solid walls, but such a design would result in prohibitive radiation damage to these walls. The use of a mirrored surface(s) partially surrounding the pellet and a laser beam that is reflected off the mirror to strike the back side of the pellet has been described in the U.S. application to M. T. Lubin, Ser. No. 57,388, filed June 17, 1970. However, there still remains potential problems of alignment, inadequate illumination of the pellet, radiation damage, etc.
Thus, there exists a need for an improved means for achieving adequate illumination of a fuel pellet in a device utilizing liquid lithium walls. This need has been met in the present invention in a manner to be described hereinbelow.